This invention relates to a shutter disc for a gate valve, and more specifically to a disc provided with plastic friction patches which make it possible to reduce the coefficient of friction between the disc and the valve body, and to its method of manufacture. Such discs are generally formed by a metal core, for example of cast iron, over which an elastomer is molded.
The coefficient of friction between the elastomers and the material constituting the body of the valve, generally cast iron covered with an epoxy resin, is relatively high, which means that after a certain number of open/close cycles of the valve substantial wear takes place on the elastomer at points whereat the guide surfaces of the disc contact the body of the valve, and there is a tearing of the material constituting the body of the valve. This has led to the appearance of friction patches or shoes adhesively bonded to the disc at the points whereat the guide surfaces contact the body of the valve. These patches are made from a plastic having a lower coefficient of friction than that of the elastomers of the material constituting the body of the valve.
Two techniques are used to fix the patches on the disc. The first comprises adhesively bonding the patches at the desired place on the metal core of the disc before covering the latter with elastomer, and the second comprises adhesively bonding the patches to the core after the elastomer has been molded on.
This method of fixing the patches, by adhesive bonding, is unsatisfactory because the discs undergo very high mechanical stresses which are incompatible with the adhesion of the bonds, the more so since they act in a liquid medium which may be hot or aggressive.
Another method of fixing the patches, without adhesive bonding, comprises using the elastomer as an intermediary between the patches and the core of the disc. In this case the patches possess, on their face turned towards the core of the disc, a plurality of cavities in swallow-tail shape which are to be filled with elastomer, thus creating a mechanical joint between the patches and the elastomer which is fixed to the core of the disc. The disadvantage of this method of fixing lies in the fact that, in order to produce a disc, it is necessary to place a core, and the patches, in the injection mold for the elastomer, leaving a free space between the patches and the core of the disc in order to allow the elastomer to pass. The elastomer is injected under a pressure in excess of 100 bars, however, which can cause the displacement of the patches in the mold, and which results in the presence of elastomer above the patches. In this case, the patches no longer fulfill their function, since friction will take place on the elastomer.